Design Study on Fully Integrated Surface-mounted Permanent Magnet Self-bearing Machines

Self-bearing permanent magnet machines, in which the stator windings produce both torque and radial levitation forces, offer many advantage over conventional machines with mechanical bearings, e.g. maintenance free, low friction losses, free rotor movement and suitability for clean and vacuum environment. In this thesis, a comprehensive research study, which encompasses topology selection, dimension optimisation, control system development and practical performance validation, is undertaken on a surface-mounted permanent magnet self-bearing machine with five degrees of freedom of motion.

A machine with 9 stator slots and 6 magnet poles is selected due to its relative uniform force capability in any arbitrary direction at all rotor angular positions. An analytical force model is developed and validated by comparison with finite element analysis. During optimisation of the design, particular attention is paid to the selection of magnet thickness and tooth body width for achieving best trade-off between motoring and bearing performance.

In order to improve the copper utilisation and machine efficiency, both the demand torque and bearing forces are produced by a single set of windings based on concentrated coils. A constrained optimisation method is developed and implemented in the control system and demonstrated in both simulations and experimental measurements.

The target performance (motoring and bearing) of the proposed self-bearing machine is estimated in the first instance by a system level simulation model which consists of an analytical force model, a mechanical dynamic model and a model of the control system.

A prototype machine is constructed and tested in both static and dynamic experiments. The static experimental rig employs rotor lift-off tests for validating the force model. In a series of dynamic experiments, a bearing stiffness of 1000N/mm is measured, which agrees with the stiffness predicted by the simulation. However, the motoring performance realised is a modest output power of 1.57kW (10Nm at 1500rpm), which is considerably lower than the target performance (which is ~6.9kW with torque rating of 22Nm at 3000rpm). The challenges and problems encountered are analysed, and several potential solutions as identified for future work.